The overall objective of these studies is to elucidate basic molecular mechanisms which regulate contractile activity of coronary vascular smooth muscle. The underlying hypothesis is that the Ca ions regulatory mechanism for actin-myosin interaction involves Ca ions dependent phosphorylation of the 16,000 dalton myosin light chains. We have shown that Ca ions dependent phosphorylation of the myosin light chains occurs in aortic native actomyosin and that the light chains are required for maintaining Ca ions sensitivity. We have also shown that cAMP and its dependent protein kinase depresses phosphorylation of the light chains and subsequent actin-myosin interaction. This suggests that contractile activity may be partly regulated directly at the level of the contractile and regulatory proteins. Our findings also suggest that the cAMP-kinase system can phosphorylate the myosin light chain kinase thereby reducing its activity and decreasing actin-myosin interaction. Relaxation of coronary arterial smooth muscle with isoproterenol was directly correlated with activation of cAMP kinase. Accordingly, that beta-mediated relaxation may involve activation of cAMP kinase, phosphorylation of the myosin light chain kinase resulting in reduced phosphorylation of the myosin light chains and consequent relaxation. Biochemical studies with isolated proteins and physiologic studies with coronary smooth muscle are directed to testing this new hypothesis.